The invention relates to a liquid-crystalline medium and to twisted nematic (TN) and supertwisted nematic (STN) liquid-crystal displays having very short response times and good steepnesses and angle dependencies, and to the novel nematic liquid-crystal mixtures used therein.
TN displays are known, for example from M. Schadt and W. Helfrich, Appl. Phys. Lett., 18, 127 (1971). STN displays are known, for example from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Congress on Liquid Crystals (8-10.04.87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. van Sprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987) and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term STN here covers any relatively highly twisted display element having a twist angle with a value of between 160xc2x0 and 360xc2x0, such as, for example, the display elements according to Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), STN-LCDs (DE-A 35 03 259), SBE-LCDs (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs (EP-A 0 246 842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
STN displays are distinguished compared with standard TN displays by significantly better steepnesses of the electro-optical characteristic line and, associated therewith, better contrast values, and by significantly lower angle dependence of the contrast.
Of particular interest are TN and STN displays having very short response times, in particular also at relatively low temperatures. In order to achieve short response times, the rotational viscosities of the liquid-crystal mixtures have hitherto been optimized using mostly monotropic additives having relatively high vapor pressure. However, the response times achieved were not adequate for every application.
In order to achieve a steep electro-optical characteristic line in the displays according to the invention, the liquid-crystal mixtures should have relatively large values for the ratio between the elastic constants K33/K11 and relatively small values for xcex94∈/∈1, where xcex94∈ is the dielectric anisotropy and ∈1 is the dielectric constant perpendicular to the longitudinal molecular axis.
In addition to optimization of the contrast and response times, further important requirements are made of mixtures of this type:
1. broad dip window
2. high long-term chemical stability
3. high electrical resistance
4. low frequency and temperature dependence of the threshold voltage.
The parameter combinations achieved are still far from adequate, in particular for high-multiplex STN displays (with a multiplex rate in the region of about 1/400), but also for medium- and low-multiplex STN displays (with multiplex rates in the region of about 1/64 and 1/16 respectively), and TN displays. This is partly attributable to the fact that the various requirements are affected in opposite manners by material parameters.
Thus, there continues to be a great demand for TN and STN displays, in particular for medium- and low-multiplex STN displays, having very short response times at the same time as a large working-temperature range, high characteristic-line steepness, good angle dependence of the contrast and low threshold voltage which meet the above-mentioned requirements.
The invention has an object of providing liquid-crystalline media, in particular for TN and STN displays, which do not have the above-mentioned disadvantages or only do so to a lesser extent and at the same time have short response times, in particular at low temperatures, and very good steepnesses. Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
It has now been found that these and other objects can be achieved if use is made of liquid-crystal mixtures which comprise one or more compounds of the formula A 
and at least one compound of the formula B 
in which
Ra and Rbb are H, an alkyl radical having from 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF3 or at least monosubstituted (i.e., up to perhalosubstituted) by halogen, it also being possible for one or more CH2 groups in these radicals to be replaced, in each case independently of one another, by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, 
xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94 in such a way that O atoms are not linked directly to one another,
Rb is an alkenyl radical having from 2 to 7 carbon atoms,
L1 is H or F, and
x is 0 or 1.
The use of the compounds of the formulae A and B in the mixtures for TN and STN displays according to the invention results in
high steepness of the electro-optical characteristic line,
low temperature dependence of the threshold voltage,
very fast response times, in particular at low temperatures.
The compounds of the formulae A and B significantly shorten, in particular, the response times of TN and STN mixtures while simultaneously increasing the steepness and reducing the temperature dependence of the threshold voltage.
The mixtures according to the invention are furthermore distinguished by the following advantages:
they have low viscosity,
they have a low threshold voltage and operating voltage, and
they effect long shelf lives in the display at low temperatures.
The invention furthermore relates to a liquid-crystal display having
two outer plates, which, together with a frame, form a cell,
a nematic liquid-crystal mixture of positive dielectric anisotropy located in the cell,
electrode layers with alignment layers on the insides of the outer plates,
a tilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0 degree to 30 degrees, and
a twist angle of the liquid-crystal mixture in the cell from alignment layer to alignment layer with a value of between 22.5xc2x0 and 600xc2x0,
a nematic liquid-crystal mixture consisting of
a) 15-75% by weight of a liquid-crystalline component A consisting of one or more compounds having a dielectric anisotropy of greater than +1.5;
b) 25-85% by weight of a liquid-crystalline component B consisting of one or more compounds having a dielectric anisotropy of between xe2x88x921.5 and +1.5;
c) 0-20% by weight of a liquid-crystalline component D consisting of one or more compounds having a dielectric anisotropy of below xe2x88x921.5, and
d) if desired, an optically active component C in such an amount that the ratio between the layer thickness (separation of the outer plates) and the natural pitch of the chiral nematic liquid-crystal mixture is from about 0.2 to 1.3,
characterized in that component A comprises at least one compound of the formula A 
and component B comprises at least one compound of the formula B 
in which Ra, Rb, Rbb, x and L1 are as defined above.
The invention also relates to TN and STN displays, in particular medium- and low-multiplexed STN displays, containing the liquid-crystal mixture according to the invention.
Formula A covers, in particular, compounds of the sub-formulae A-1 to A-4
in which alkyl is a straight-chain or branched alkyl radical having from 1 to 12 carbon atoms, and alkenyl is a straight-chain or branched alkenyl radical having from 2 to 12 carbon atoms.
Particular preference is given to mixtures according to the invention which comprise at least one compound of the formula A-1 and/or A-2, particularly preferably in each case at least one compound of the formula A-1 in which L1=F.
In the formulae A, Ra (and alkyl or alkenyl in formulae A-1 to A4) is particularly preferably straight-chain alkyl or alkoxy, 1E-alkenyl or 3E-alkenyl having from 2 to 7 carbon atoms.
Formula B covers, in particular, compounds of the sub-formulae B-1 to B-6
in which
alkyl are each, independently of one another, a straight-chain or branched alkyl radical having from 1 to 12 carbon atoms, and
alkenyl and
alkenyl* are each, independently of one another, a straight-chain or branched alkenyl radical having from 2 to 12 carbon atoms.
The radicals xe2x80x9calkylxe2x80x9d and xe2x80x9calkenylxe2x80x9d or xe2x80x9calkenyl*xe2x80x9d are preferably straight-chain and have up to 7 carbon atoms.
The use of compounds of the formulae A and B in the liquid-crystal mixtures according to the invention results in particularly low rotational viscosity values and in TN and STN displays having high steepness and fast response times, in particular at low temperatures. For example, the threshold voltages of the inventive media in STN-displays with 240xc2x0 twist and 3xc2x0 tilt are preferably in the range from 1.0 V to 2.0 V, particularly preferably in the range from about 1.1 V to 1.8 V. The values of the steepness of the electrooptical response of the media in the STN-displays makes them suitable for medium having higher multiplex ratios. Typically they can be used by multiplex ratios of up to 1/128 and even up to 1/200 or more and even up to 1/400. Preferably the media with the lower threshold voltages are used with a multiplex ratio of 1/64 up to 1/128. The low values of the rotational viscosity of the media lead to fast response times. The response times in the STN-displays, as mentioned, are preferably in the range from about 60 ms to 300 ms, when measured with a multiplex ratio of 1/64 and a bias of 1/9. More preferably, the response times are in the range from 90 ms to 280 ms. The higher response times are observed for the media with the lower threshold voltages and vice versa.
Besides the compounds of the formula A, component A of the liquid-crystalline mixture according to the invention additionally preferably comprises one or more 3,4,5-trifluorophenyl compounds selected from the group consisting of the compounds of the formulae IIa to IIl 
Besides the compounds of the formulae A and B, the medium according to the invention may additionally comprise one or more compounds containing polar end groups, for example, those of the formulae II*a to II*s 
in which R2 is as defined for Ra, and L3 and L4 are each, independently of one another, H or F. R2 in these compounds is particularly preferably alkyl, alkenyl, alkoxy or alkenyloxy having up to 7 carbon atoms.
The medium according to the invention or component A particularly preferably comprises compounds of the formulae IIa, IIb, IIc, IId, IIe, IIf, IIg, IIj, II*b, II*c, II*d, II*f and/or II*i, in particular compounds of the formulae IIa, IIb, IId, IIi, II*a and II*i.
The mixture according to the invention preferably further comprises one or more cyano compounds of the formulae IIIa to IIIj: 
in which R3 is as defined for Ra, and L1, L2 and L5 are each, independently of one another, H or F. R3 in these compounds is particularly preferably alkyl, alkenyl or alkoxy having up to 7 carbon atoms.
Particular preference is given to mixtures which comprise one or more compounds of the formulae IIIb, IIIc and IIIf, in particular those in which L1 and/or L2 is F.
Preference is furthermore given to mixtures which comprise one or more compounds of the formula IIIf and/or IIIg in which L2 is H and L1 is H or F, in particular F.
The individual compounds of the formulae A, B and of the formulae IIa-IIl, II*a-II*s and IIIa to IIIj and their sub-formulae or also other compounds which can be used in the mixtures or TN and STN displays according to the invention are either known or can be prepared analogously to the known compounds.
The compounds of the formula B have low viscosities, in particular low rotational viscosities, and low values for the ratio of the elastic constants K33/K11, and therefore result in short response times in the displays according to the invention, while the presence of compounds of the formula A of high dielectric anisotropy, in particular in increased concentrations, causes a reduction in the viscosity.
Preferred liquid-crystal mixtures comprise one or more compounds of component A, preferably in a proportion of from 15% to 75%, particularly preferably from 20% to 65%. These compounds have a dielectric anisotropy xcex94∈xe2x89xa7+3, in particular xcex94∈xe2x89xa7+8, particularly preferably xcex94∈xe2x89xa7+12.
Further preferred mixtures comprise
one or more, in particular one or two, compounds of the formula A,
one, two or three compounds of each of the formulae A-1 and A-3,
one or more, in particular two, three or four, compounds of the formula B,
one or more, in particular from two to five, compounds of the formulae IIIb, IIIc and/or IIIf.
Preferred liquid-crystal mixtures comprise one or more compounds of component B, preferably from 25 to 85%. The compounds from group B are distinguished, in particular, by their low values for the rotational viscosity xcex31.
Component B preferably further comprises one or more compounds of the formula IV 
in which
m is 0 or 1,
R4 is an alkenyl group having from 2 to 7 carbon atoms,
R5 is defined for Ra in the case where m=0 and is F, Cl, CF3 or OCF3 in the case where m=1,
L1 and L2 are each independently of one another, H or F.
Particularly preferred compounds of the formula IV are those in which R4 is alkenyl having from 2 to 7 carbon atoms, in particular those of the following formulae: 
in which R3a and R4a are each, independently of one another, H, CH3, C2H5 or n-C3H7, and alkyl is an alkyl group having from 1 to 7 carbon atoms.
Particular preference is given to TN and STN displays according to the invention in which the liquid-crystal mixture comprises at least one compound of the formula IV1 and/or IV3 in which R3a and R4a each have the same meaning, and displays in which the liquid-crystal mixture comprises at least one compound of the formula IV5.
In a further preferred embodiment, the mixtures according to the invention comprise one or more compounds of the formula IV6.
Component B preferably furthermore comprises compounds selected from the group consisting of the bicyclic compounds of the formulae V-1 to V-9
and/or one or more compounds selected from the group consisting of the tricyclic compounds of the formulae V-10 to V-26
in which R6 and R7 are as defined for Ra in the formula A, and L is H or F.
Particular preference is given to compounds of the formulae V-24 to V-32 in which R6 is alkyl and R7 is alkyl or alkoxy, in particular alkoxy, each having from 1 to 7 carbon atoms. Preference is furthermore given to compounds of the formulae V24, V-26 and V-32 in which L is F.
R6 and R7 in the compounds of the formulae V-1 to V-32 are particularly preferably straight-chain alkyl or alkoxy having from 1 to 12 carbon atoms.
If desired, the liquid-crystalline mixtures comprise an optically active component C in such an amount that the ratio between the layer thickness (separation of the outer plates) and the natural pitch of the chiral nematic liquid-crystal mixture is greater than 0.2. A multiplicity of chiral dopants, some of which are commercially available, is available to the person skilled in the art for the component, such as, for example, cholesteryl nonanoate, S-811, S-1011 and S-2011 from Merck KGaA, Darmstadt, and CB15 (BDH, Poole, UK). The choice of dopants is not crucial per se.
The proportion of the compounds of component C is preferably from 0 to 10%, in particular from 0 to 5%, particularly preferably from 0 to 3%.
The mixtures according to the invention may also, if desired, comprise up to 20% of one or more compounds having a dielectric anisotropy of less than xe2x88x922 (component D).
If the mixtures comprise compounds of component D, these are preferably one or more compounds containing the structural unit 2,3-difluoro-1,4-phenylene, for example compounds as described in DE-A 38 07 801, 38 07 861, 38 07 863, 38 07 864 or 38 07 908. Particular preference is given to tolans containing this structural unit, as described in the International Patent Application PCT/DE 88/00133.
Further known compounds of component D are, for example, derivatives of the 2,3-dicyanohydroquinones or cyclohexane derivatives containing the structural unit 
as described in DE-A 32 31 707 and DE-A 34 07 013.
The liquid-crystal displays according to the invention preferably contain no compounds of component D.
The term xe2x80x9calkenylxe2x80x9d in the definition of Ra, Rb, Rbb, R2, R3, R4, R5, R6 and R7 covers straight-chain and branched alkenyl groups, in particular the straight-chain groups. Particularly preferred alkenyl groups are C2-C7-1E-alkenyl, C4-C7-3E-alkenyl, C5-C7-4-alkenyl, C6-C7-5-alkenyl and C7-6-alkenyl, in particular C2-C7-1E-alkenyl C4-C7-3E-alkenyl and C5-C7-4-alkenyl.
Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5 carbon atoms are generally preferred.
Further preferred embodiments relate to liquid-crystal mixtures according to the invention which
additionally comprise one or more, particularly preferably one, two or three, heterocyclic compounds of the formula Va and/or Vb 
in which
R6 and R7 are as defined above, and
Y is F or Cl,
where the proportion of the compounds from the group consisting of Va and Vb is preferably from 2 to 35%, in particular from 5 to 20%,
additionally comprise one or more, particularly preferably one, two or three, tolan compounds of the formulae T2a, T2b and/or T2c 
in which R6 and R7 are as defined above.
The proportion of the compounds from the group consisting of T2a, T2b and/or T2c is preferably from 2 to 40%, in particular from 5 to 30%. The mixture according to the invention preferably comprises two or three compounds of the formula T2a and/or T2b.
In particularly preferred embodiments, the mixtures comprise
at least one compound of the formula IIIc;
at least one tolan compound of the formula T2c;
at least two, in particular three, compounds of the formula B;
at least one compound of the formula T2a and/or at least one compound of the formula T2b;
at least one compound of the formula 
in which L1 is H or F;
at least 2.5% by weight of the compounds of the formula IV5;
5-30% by weight, preferably 10-25% by weight, of the compounds of the formula A;
5-30% by weight, preferably 5-20% by weight, of the compounds of the formula B;
at least one compound of the formula A in which Ra is CH3, C2H5, n-C3H7 or n-C5H11.
Further particularly preferred embodiments relate to liquid-crystal mixtures which comprise
a total of from three to six compounds of the formulae A and B, where the proportion of these compounds with respect to the mixture as a whole is from 25 to 65%, in particular from 30 to 55%,
more than 20% of compounds of positive dielectric anisotropy, in particular having xcex94∈xe2x89xa7+12.
The mixtures according to the invention are distinguished, in particular on use in TN and STN displays of high layer thicknesses, by very low total response times (ttot=ton+toff).
The liquid-crystal mixtures used in the TN and STN cells according to the invention are dielectrically positive, with xcex94∈xe2x89xa7+1. Particular preference is given to liquid-crystal mixtures with xcex94∈xe2x89xa7+3, in particular with xcex94∈xe2x89xa7+5.
The liquid-crystal mixtures according to the invention have favorable values for the threshold voltage V10/0/20 and for the rotational viscosity xcex31. If the value for the optical path difference dxc2x7xcex94n is pre-specified, the value for the layer thickness d is determined by the optical anisotropy xcex94n. In particular at relatively high values for dxc2x7xcex94n, the use of liquid-crystal mixtures according to the invention having a relatively high value for the optical anisotropy is generally preferred, since the value for d can then be selected to be relatively small, which results in more favorable values for the response times. However, liquid-crystal displays according to the invention which contain liquid-crystal mixtures according to the invention with smaller values for xcex94∈ are also characterized by advantageous values for the response times.
The liquid-crystal mixtures according to the invention are furthermore characterized by advantageous values for the steepness of the electro-optical characteristic line, and can be operated with high multiplex rates, in particular at temperatures above 20xc2x0 C. In addition, the liquid-crystal mixtures according to the invention have high stability and favorable values for the electrical resistance and the frequency dependence of the threshold voltage. The liquid-crystal displays according to the invention have a large working-temperature range and good angle dependence of the contrast.
The construction of the liquid-crystal display elements according to the invention from polarizers, electrode base plates and electrodes having a surface treatment such that the preferential alignment (director) of the liquid-crystal molecules in each case adjacent thereto is usually twisted by a value of from 160xc2x0 to 720xc2x0 from one electrode to the other corresponds to the usual structure for display elements of this type. The term xe2x80x9cusual structurexe2x80x9d here is broadly drawn and also covers all derivatives and modifications of the TN and STN cell, in particular also matrix display elements and display elements containing additional magnets.
The surface tilt angle at the two outer plates may be identical or different. Identical tilt angles are preferred. Preferred TN displays have pre-tilt angles between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0xc2x0 to 7xc2x0, preferably from 0.01xc2x0 to 5xc2x0, in particular from 0.1xc2x0 to 2xc2x0. In the STN displays, the pre-tilt angle is from 1xc2x0 to 30xc2x0, preferably from 1xc2x0 to 12xc2x0 and in particular from 3xc2x0 to 10xc2x0.
The twist angle of the TN mixture in the cell has a value of between 22.5xc2x0 and 170xc2x0, preferably between 45xc2x0 and 130xc2x0 and in particular between 80xc2x0 and 115xc2x0. The twist angle of the STN mixture in the cell from alignment layer to alignment layer has a value of between 100xc2x0 and 600xc2x0, preferably between 170xc2x0 and 300xc2x0 and in particular between 180xc2x0 and 270xc2x0.
The liquid-crystal mixtures which can be used in accordance with the invention are prepared in a manner which is conventional per se. In general, the desired amount of the components used in lesser amount are dissolved in the components making up the principal constituent, advantageously at elevated temperature. It is also possible to mix solutions of the components in an organic solvent, for example in acetone, chloroform or methanol, and to remove the solvent again, for example by distillation, after thorough mixing.
The dielectrics may also comprise further additives which are known to the person skilled in the art and are described in the literature. For example, 0-15% of pleochroic dyes may be added.
The entire disclosures of all applications, patents and publications, cited above or below, and of corresponding German application No. 101 63 271.1, filed Dec. 21, 2001, is hereby incorporated by reference.
In the present application and in the examples below, the structures of the liquid-crystal compounds are indicated by means of acronyms, the transformation into chemical formulae taking place in accordance with Tables A and B. All radicals CnH2n+1 and CmH2m+1 are straight-chain alkyl radicals having n and m carbon atoms respectively (n and m: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12). The alkenyl radicals have the trans-configuration. The coding in Table B is self-evident. In Table A, only the acronym for the parent structure is indicated. In individual cases, the acronym for the parent structure is followed, separated by a dash, by the code indicated in the table below for the substituents R1*, R2*, L1*, L2* and L3*.
The TN and STN displays preferably contain liquid-crystalline mixtures composed of one more compounds from Tables A and B.